The invention pertains to cathodic electrocoating processes and particularly to an improved cathodic electrocoating composition and process based on the addition of peroxide to the electrocoating bath.
Non-uniformity of phosphate coatings on steel and the presence of pores, especially when no chrome sealer was used, present problems in electrocoating. The uneven current distribution due to conductivity differences in the metal surface affects coating uniformity. This is especially true of coatings with poor coalescing properties (latices). Formation of ridges is not uncommon for these types of coatings. With solution polymers, gassing in the cathode will cause this coating non-uniformity. These problems lead to poor appearance and coating performance. In cathodic electrodeposition, the evolution of hydrogen gas at the cathode is known to cause foaming and the formation of pores and pits in the deposited coating. This gives rise to the formation of non-uniform coatings that have poor resistance to chemical attack because of voids present in the coating. Appearance and performance properties are both important for primers and one coat systems.
It now has been found that the addition of oxidizers, especially hydrogen peroxide, to cathodic electrocoating compositions substantially improves the properties of such electrocoated and cured paint films. The incorporation of hydrogen peroxide solutions into the cathodic electrocoating baths avoids the evolution of hydrogen gas at the cathode and avoids the formation of pores and pits in the film. With improvements in coating uniformity due to the absence of trapped gases, contact between the metal and the coating is improved and thus the adhesion. Improvement in salt spray and detergent resistance follows improvement in adhesion. The incorporation of a 30% hydrogen peroxide solution was found to improve coating appearance and performance. These improvements were found in both cationic latices and solution polymers when tested at levels dependent on the demands of the formulations. Incorporation is by direct addition to either the electrocoat bath or to the resin or pigment components. A further improvement in the electrocoating process was realized in that iron does not dissolve in the electrocoating bath. Appearance improvements are smoothness, gloss, color, and pretreatment telegraphing. Performance improvements are salt spray resistance and adhesion. Further advantages of this invention include uniform coating appearance, improved gloss, minimizes staining and yellowing of baked coatings, improved coating adhesion to substrates, uniform coating appearance over both chrome and non-chrome rinsed substrates, and improved detergent resistance. These and other advantages of this invention shall become more apparent by referring to the detailed description of the invention and the illustrative examples.